For flat parts, like printed circuit boards and silicon wafers, optical inspection systems operate nicely in a three axis configuration. The X and Y axes are used to position the workpiece relative to a camera or a sensor and a Z axis provides focus. The camera used with an optical inspection system needs to be aimed squarely at the feature to be inspected or parallax imaging distortions will cause measurement errors. For workpieces that are generally cylindrical, like a medical stent or a machined piston, mounting the part on a rotary or fourth axis provides the camera with a straight on view of any area of the part. However, when a part has complex contours like, for example, a bullet, obtaining a high resolution image at each point on the surface while aiming the camera normal to a surface patch requires a five axis of motion optical inspection system.
Traditional five axis motion systems used with optical inspection have drawbacks that make them less than ideal. One approach has the part under inspection mounted on a rotary table and then this rotary table is mounted on a second rotary table to obtain the fifth or tilting axis. A drawback is that the amount of mass that the tilting rotary is moving is significantly higher than that of just the part. This makes inspection operations slower than that of a four axis inspection system. It also makes lighting the part for optical inspection more challenging as the mass of the dual rotaries can now preclude convenient placement of lighting components. Five axis motion systems of this type produced by Optical Gauging Products of Rochester, N.Y., USA and Werth Messtechnik, of Giessen, Germany.
An alternative approach to the fifth tilting axis is to mount the camera and lens components of the inspection system on a rotational stage which is in-turn attached to a Z-axis. One drawback with this approach is that increased mass is added to one of the existing stage axes. A further drawback is that the length of the optical track from object to image at the camera must be accommodated by the stage travel of the system. This can add significant size and cost to the system.